feat(datepicker): add theming support

[crisbeto]

Adds support for theming the datepicker using the color attribute, as well as inheriting the theme color from the form field around a datepicker input.

[jelbourn]

@crisbeto @mmalerba why is this major instead of minor release?

[mmalerba]

@crisbeto needs rebase

[crisbeto]

Rebased.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}